Conventional high voltage power systems in which alternating current is provided by means of a high voltage power transformer are severely limited in flexiblity of secondary outputs. When a plurality of secondaries are to be operated from a single transformer primary, a single alternating current frequency is produced at each of the secondaries. Conventional high voltage power transmission systems make no provision for the generation of a plurality of different alternating current frequencies from the different secondaries of a single transformer. Consequently, to achieve different alternating current frequencies to different parts of a system utilizing the high voltage power supply, separate frequency transformers are required. These transformers involve a great deal of expense, especially where a number of different high voltage alternating current frequencies are required within the system. Each different transformer thus produces a power output, separately regulated, and at a unique and predetermined frequency.
The use of power transistors to impress different alternating current frequencies on separate voltage taps from different secondaries of a single transformer have proven to be unsatisfactory. A power transistor in such an arrangement conducts current at intervals regulated by increases and decreases of voltage at the base of the transistor. However, to operate in this manner power transistors must be connected in series with the electrical loads to which alternating current is to be provided. Under such conditions, the power transistors tend to break down rapidly, especially at voltages in excess of two kilovolts. In xerographic reproduction devices, voltages of from two to six kilovolts are normally utilized to provide power to corotrons to set up electrostatic fields in connection with image reproduction processes.
It is an object of the present invention to provide an arrangement for impressing an alternating current of a selected frequency and waveform on secondary outputs of a high voltage transformer without incurring the disadvantages of prior systems. This is possible because the present invention interposes a light dependent resistor in series with the power leads to the load to which electric current is to be supplied. Such light dependent resistors unlike power transistors, are able to accomodate voltages of from two to six kilovolts without adverse effect.
A further object of the invention is to provide a system for producing an alternating current in a high voltage power supply in which the frequency generating components are electrically isolated from the high voltages to be transmitted to the electrical load. This feature of electrical isolation is a highly desireable characteristic achieved using the present invention, since the resistance in the series connections to the load are governed by the illuminating effect of light emitting diodes, which are in optical communication with the light dependent resistors, but in electrical isolation therefrom. Thus, a much wider range of frequency generating components, such as operational amplifiers including differential amplifier circuits, is available for use, since these components are not subject to the high voltages to be provided to the electrical load. The electrical isolation of the frequency generating components allows electrical components of a much finer degree of precision to be used to effectuate the provision of alternating current to the load than is possible using conventional devices coupled in the electrical power circuitry.
In a xerographic reproduction device having a high frequency, low current, high voltage alternating current power supply, where a significantly lower frequency is required at the load, the desired lower frequency can be derived by sensing voltage or current at the corotron plate or shield. The signal so derived can be used in conjunction with a reference signal to generate a desired lower frequency control signal. This control signal in turn is used to periodically actuate a light emitting diode to regulate the power output to the corotron wire through a light dependent resistor connected in series therewith in accordance with the desired frequency. For example, if the desired frequency is four hertz and the load is fed from a rectified and filtered 60 hertz power supply, the four hertz signal can be impressed upon the DC signal without noticeable distortion. By employing a programmable feedback circuit, electromagnetic frequency transformation is not required. Instead of a conventional transformer, the resistance of a controlled light dependent resistor is used to vary the voltage provided to the electrical load.
Control of the resistance of the light dependent resistors allows any wave shape of alternating current to be provided to the load. Also, any number of alternating current and direct current voltage levels and outputs can be produced from a single transformer and rectifier by separately programming the illumination of the several light dependent resistors operated in parallel from a single transformer.
As previously noted the present invention has particular utility in connection with xerographic reproduction devices. One form of the corotron of such device is shown in U.S. Pat. No. 2,836,725. In the corotron depicted therein, a conductive corona electrode is provided in the form of an elongated wire connected to a direct current power supply. The wire is partially surrounded by a grounded electrically conductive shield. The wire is used to produce an electrostatic charge on another circuit element called a plate, which usually takes the form of a rotatable drum in a xerographic reproduction device. The drum is spaced from the wire so that a clear pathway of electron flow is provided both between the wire and the shield and between the wire and the drum. A corona discharge current flows partially between the wire and the drum and partially between the wire and the shield. According to the present invention, an alternating current is developed to effectuate a cyclic potential variation of the wire with respect to the drum and to the shield. This alternating current may be derived from direct current voltages, such as are developed at the transformer secondaries and utilized in connection with a xerographic reproduction device.
An alternative form of corotron may be biased in a manner taught in U.S. Pat. No. 2,879,395. In this patent, an alternating current corona generating potential is applied to the corotron wire and a direct current potential is applied to the conductive shield partially surrounding the wire to regulate the flow of ions from the wire to the drum. Other corotron configurations also exist, but need not be described in detail as the present invention is applicable to virtually any such device which employs a high voltage power supply to the corotron elements.